Ecology

Ectotherm and Endotherm

Endotherms generate most of the heat they need internally. When it's cold out, they increase metabolic heat production to keep their body temperature constant. Because of this, the internal body temperature of an endotherm is more or less independent of the temperature of the environment.

The sum total of the biochemical reactions that take place in an organism are called its metabolism. Metabolic reactions involve breaking down fuel molecules, such as sugars, and using the energy stored in them to do work. The processes that convert energy stored in food molecules into biological work are not very efficient, so heat is generated as a byproduct.

The higher an organism's metabolic rate—the amount of chemical fuel it burns in a given period of time—the more heat it will produce.

So, as an endotherm is exposed to colder external temperatures, it will increase its metabolic rate, burn more fuel, and produce extra heat to keep its body temperature constant.

This pattern is shown on the graph below: the mouse maintains a steady body temperature close to 37^o C

A graph of a mouse's internal temperature across different outside temperatures.

X axis: outside temperature in degrees Celsius, 0 to 40 degrees

Y axis: animal's internal temperature in degrees Celsius, 0 to 40 degrees

The mouse's body temperature stays close to 37 degrees Celsius across a range of temperatures approximately 5 degrees Celsius to 42 degrees celsius, with a downturn below 5 degrees Celsius and an upturn above 42 degrees Celsius. That is, it is a straight horizontal line at 37 degrees Celsius for most of the external temperature range.

A mouse is an endotherm; it generates metabolic heat to maintain internal body temperature.

For ectotherms, on the other hand, body temperature mainly depends on external heat sources. That is, ectotherm body temperature rises and falls along with the temperature of the surrounding environment. Although ectotherms do generate some metabolic heat—like all living things—ectotherms can't increase this heat production to maintain a specific internal temperature.

A graph of a snake's internal temperature across different outside temperatures.

X axis: outside temperature in degrees Celsius, 0 to 40 degrees

Y- axis: animal's internal temperature in degrees Celsius, 0 to 40 degrees

The snake's body temperature varies with external temperature, creating a line with a slope of one between about 5 degrees Celsius and 42 degrees Celsius.

A snake is an ectotherm; it's body temperature changes with the temperature of its environment.

Most ectotherms do regulate their body temperature to some degree, though. They just don't do it by producing heat. Instead, they use other strategies, such as behavior—seeking sun, shade, etc.—to find environments whose temperature meets their needs.